Optical wavelength-division multiplexing (WDM) elements are becoming increasingly important in advanced optical communications networks incorporating optical fiber transmission paths. Silica optical fiber has a transmission bandwidth of over 300 terahertz per second. Such an extremely large bandwidth is, however, limited by the electronics on the transmitting and receiving ends. Such electronic transmitters and receivers, typically bases on silicon electronics, are limited commercially at the present time to 2 to 10 gigabits/s (Gbs). Further increases to 40 Gbs are contemplated, but further increases will be difficult to achieved.
For these reasons, WDM has been proposed in which multiple (N) electronic data channels, as illustrated in FIG. 1, enter a transmitter 10 and modulate separate optical emitters such as lasers 12 having N respective output carrier wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N. Conveniently, these wavelengths are arranged in a WDM wavelength comb having the neighboring wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N separated by a substantially constant inter-channel spacing given by EQU .DELTA..lambda..sub.S =.lambda..sub.i+1 -.lambda..sub.i. (1)
An optical wavelength-division multiplexer 14 combines the optical signals of different wavelengths and outputs the combined signal on a single optical fiber 16. An optical receiver 20 includes a wavelength-division demultiplexer 22 which divides its received signals according to their optical wavelength to N optical detectors 24 according to the same wavelength allocation .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N. In view of usually experienced reciprocity in passive systems, a wavelength-division demultiplexer is usually substantially identical to a wavelength-division multiplexer with a reversal of their inputs and outputs.
Additionally, an optical add/drop multiplexer (ADM) 30 may be interposed on the optical path 16 between the transmitter and the receiver 20. The optical add/drop circuit 30 removes from the optical channel on the fiber 16 one or more wavelength channels at wavelength .lambda..sub.AD and inserts back onto the fiber 16 an optical data signal perhaps containing different information but at the same optical carrier wavelength .lambda..sub.AD. The ADM 30 is typically implemented with technology closely resembling the WDMs 14, 22. All-optical networks have been proposed in which a distributed networks having many nodes each including a transmitter 10 and receiver 20 are linked by a functionally passive network which routes the signals between the nodes according to their wavelengths. The routing elements in such an all-optical network require switching elements similar to the ADM 30.
In order to maximize the transmission capacity of the optical fiber 16, the wavelength channels .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N should be placed as closely together as possible with a minimum channel spacing .DELTA..lambda..sub.S. In advanced systems, this inter-channel spacing .DELTA..lambda..sub.S is 1 nm or less for a signal centered around 1300 or 1550 nm, the preferred bands for silica fiber. Such closely spaced WDM networks are referred to as dense WDM networks (DWDM).
The network design described above may be subject to a problem arising from the fact that the operation of the transmitter 10, receiver 20 and intermediate node 30 are all referenced to the same set of WDM wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N. However each of the distributed elements must provide its own wavelength calibration. Due to environmental and aging effects, the wavelength calibration settings at one element are likely to differ from those at another element. In view of the close spacing of the optical channels, any miscalibration between network elements is likely to produce inter-channel interference.
For an optimized optical system, the fiber 16, the WDMs 14, 22, and the ADM 30 are typically designed to be single-mode at least at their ports for the optical wavelengths being used. Although each of the lasers 12 is likely emitting light across an exceedingly narrow bandwidth, the single-mode response of the frequency sensitive elements 14, 22, 30 usually has a wavelength (frequency) characteristic that approximates a gaussian distribution about the center wavelength .lambda..sub.0 of the channel F(.lambda.)=exp(-(.lambda.-.lambda..sub.o.sup.2)/.DELTA..lambda..sub.G. sup.2). The value of the gaussian passband .DELTA..lambda..sub.G can be fairly freely chosen for present day fabrication techniques. However, the value of the passband is subject to countervailing restraints. For dense WDM systems, the inter-channel spacing .DELTA..lambda..sub.S is made as small as possible. The gaussian passband .DELTA..lambda..sub.G must be substantially smaller than the inter-channel spacing .DELTA..lambda..sub.S to avoid interference between channels. On the other hand, the frequency characteristics of the lasers 12 and other frequency-sensitive elements are subject to permanent or temporary variations. If the passband .DELTA..lambda..sub.G is made too small, the peak is very narrow and small variations in wavelength away from the peak's wavelength .lambda..sub.0 causes operation to shift to the sides of the peak, thereby degrading the signal strength. That is, for a strong signal the passband .DELTA..lambda..sub.G should be made as large as possible to provide a broad top of the peak.
Amersfoort et al. have already recognized these problems, as disclosed in U.S. Pat. No. 5,629,992. These patents describe arrayed waveguide gratings, also called phasars, of the sort described by Hunsperger et al. in U.S. Pat. No. 4,773,063, and by Dragone in U.S. Pat. Nos. 5,412,744 and 5,488,680. In particular Amersfoort et al. describe a WDM phasar 40 exemplified in the schematic illustration of FIG. 2. A single-mode waveguide 42 is coupled to one end of a multi-mode waveguide 44 of length chosen to produce a doubled image of the radiation from the single-mode waveguide 42 at a port 46 on one side wall 47 of a first free space region 48. The multi-mode waveguide 44 acts as a multi-mode interferometer (MMI). Multiple single-mode array waveguides 50 are coupled to ports on the other side of the first free space region 48 in the form of a star coupler. The array waveguides 50 are coupled on the other end to one side of a second free space region 52. The array waveguides 50 have lengths with predetermined length differences between them to act as an arrayed waveguide grating (AWG), operating similarly to a planar diffraction grating. Single-mode output waveguides 54 are coupled to the other side of the second free space region 50 along an output wall 56. The AWG causes the multi-wavelength signal from the input waveguide 42 to be wavelength demultiplexed on the respective output waveguides 54. Because of the reciprocal nature of the device, the roles of input and output can be reversed so that the same structure can be used as a wavelength multiplexer and as a wavelength demultiplexer. The placement and number of waveguides contemplated by Amersfoort et al. are wider than the example of a single input presented below.
The gaussian wavelength distribution described above for single-mode elements is related to the gaussian spatial distribution of intensity experienced at the outputs of single-mode fibers. However, the multi-mode waveguide 44, because it typically contains two closely spaced peaks at the port 46, produces a spatial output pattern into the first free space region 48 that is not gaussian but is much flatter at its peak than a corresponding gaussian distribution of the same passband. The wavelength characteristic of the free space between the multi-mode waveguide 44 and the rest of the phasar 40 is therefore also flattened. As a result, with the use of the multi-mode interference filter 44, it is possible to obtain a narrow wavelength response for the phasar but with smaller variations in response for small wavelength variations about the central values. However, the MMI solution of Amersfoort et al. suffers a power penalty of 2 to 3 dB as the single-mode power is spread out over a wider area. Chen discloses a somewhat similar approach in U.S. Pat. No. 5,889,906, wherein he uses multi-mode sections, not in order to flatten the bandpass of the individual channels as Amersfoort et al. did, but in order to obtain better uniformity for the different individual channels.
Dragone in U.S. Pat. No. 5,412,744 broadens the passband of a standard phasar by having a Y-coupler interposed between the single-mode input waveguide 42 and two single-mode waveguides separately coupled into the free space region 48. The result is to spread the intensity for one mode across a larger area on the input wall of the free space region 48. This approach suffers a similar power penalty of 2 to 3 dB.
Dragone in U.S. Pat. No. 5,488,680 suggests the advantage of cascading wavelength routing devices such as phasars. One configuration he develops includes a Mach-Zehnder interferometer (MZI), a 3 dB cross coupler between the two output waveguides of the MZI, and a standard phasar having a first free space region receiving the two waveguides from the MZI on its input wall. The geometry is such that one output waveguide focuses radiation of one wavelength at the output of the phasar and the other output waveguide radiation of another wavelength there with about 0.9 dB ripple for wavelengths in between. Thereby, the passband of the combination of the Mach-Zehnder and the phasar is flattened.
Thompson et al. disclose an alternative technique for passband flattening of a phasar in "An original low-loss and pass-band flattened SiO.sub.2 on Si planar wavelength demultiplexer," OFC '98 Technical Digest, Optical Fiber Conference, Feb. 22-27, 1988, San Jose, Calif., p. 77. Two phasars are arranged in series. The first phasar has a free spectral range equal to the channel spacing. The free spectral range is the frequency range over which the frequency characteristics are repeated. In most one-stage phasar designs, all N channel spacings fit within one free spectral range. While the Thompson design theoretically offers a lossless broadening, in practice phasars are difficult to build to achieve optimum performance.
Accordingly, it is desired to provide a phasar design which offers passband flattening with low loss in a simple design.